📝 Update global-shortcut API document.

docs/api/global-shortcut.md

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+# global-shortcut
+
+The `global-shortcut` module can register/unregister a global keyboard shortcut
+in operating system, so that you can custom the operations for various shortcuts.
+Note that it is global, even the app does not get focused, it still works.
+
+```javascript
+var globalShortcut = require('global-shortcut');
+
+// Register a 'ctrl+x' shortcut listener.
+var ret = globalShortcut.register('ctrl+x', function() { console.log('ctrl+x is pressed'); })
+if (!ret)
+  console.log('registerion fails');
+
+// Check whether a shortcut is registered.
+console.log(globalShortcut.isRegistered('ctrl+x'));
+
+// Unregister a shortcut.
+globalShortcut.unregister('ctrl+x');
+
+// Unregister all shortcuts.
+globalShortcut.unregisterAll();
+```
+
+## globalShortcut.register(keycode, callback)
+
+* `keycode` String
+* `callback` Function
+
+Registers a global shortcut of `keycode`, the `callback` would be called when
+the registered shortcut is pressed by user.
+
+`keycode` is a string to specify shortcut key, such as "ctrl+shift+a".
+
+A `keycode` consists of modifier and key two parts:
+
+__Modifiers__: control(ctrl), command(cmd), alt, shift, commandorcontrol(cmdorctrl).
+
+__Supported keys__: 0-9, a-z, up, down, left, right, home, end, pagedown, pageup,
+insert, delete, esc, space, backspace, tab, f1-f12, volumeup, volumedown, media 
+keys(medianextrack, mediaprevioustrack, mediastop, mediaplaypause).
+
+## globalShortcut.isRegistered(keycode)
+
+* `keycode` String
+
+Return whether the shortcut is registered.
+
+## globalShortcut.unregister(keycode)
+
+* `keycode` String
+
+Unregisters the global shortcut of `keycode`.
+
+## globalShortcut.unregisterAll()
+
+Unregisters all the global shortcuts.